Name __________________________________________________ Date ________________________
NORTH ALLEGHENY SENIOR HIGH SCHOOL
AP Chemistry
GRAVIMETRIC ANALYSIS: PERCENTAGE PURITY IN A SAMPLE OF SILVER
INTRODUCTION:
An alloy contains two or more metals. Many silver alloys are composed of silver and copper. The
mass of silver in a sample of an alloy may be determined by isolating the silver in the form of a pure silver
compound and weighing the compound. The first step in such an analysis is to obtain a solution of copper
and silver ions, using a suitable reagent such as nitric acid. A silver-copper alloy reacts with nitric acid and
forms a bluish-solution which contains silver ions and copper ions. The hydrated copper(II) ions are
responsible for the blue color of the solution.
The next step is to add a reagent to the solution which forms a relatively insoluble silver compound,
but does not react with the copper ions. Any soluble chloride, for example sodium chloride, reacts with silver
ions and forms insoluble silver chloride but does not react with copper(II) ions. The precipitated silver
chloride may then be filtered out of the solution, dried, and weighed. The mass of the silver in the original
alloy sample may then be determined by calculating the mass of the silver in the weighted precipitate of NaCl.
In this experiment your objective is to determine the percentage purity of a sample of sterling silver,
compare it to the accepted percentage of sterling silver, and to become acquainted with the procedures and
calculations of gravimetric analysis.
SAFETY ALERTS:
1. Nitric acid is an extremely corrosive substance that is damaging to skin and eyes. Use great care when
handling it.
2. As the silver-copper alloy dissolves in the nitric acid (see procedure), the gases nitrogen monoxide,
NO, and nitrogen dioxide, NO2, are evolved (see appendix A at end of lab). Both gases are toxic, so
this reaction must be carried out in a fume hood.
PRE-LAB ASSIGNMENT:
Along with the introduction and procedure, set up the calculation as described in step 3 of the
procedure below. Setting this up now will allow you to plug in your mass and obtain the amount of
sodium chloride needed quickly, making the lab continuous.
PROCEDURE:
1. Obtain the mass of the silver alloy given to you in class.
2. Dissolve the Silver: Put the alloy in a clean 100-mL beaker, and carefully pour 10 mL of 6 M nitric acid
over it (SEE SAFETY ALERT 2). Cover the beaker with a watch glass and relocate the beaker to
the fume hood. Place the beaker on a hot plate and gently heat the solution so that the alloy
dissolves. This process will take about 10-15 minutes so proceed on to step 3. Allow the alloy to
totally dissolve.
3. Precipitate the Silver: Calculate the amount of sodium chloride that would be necessary to precipitate
the silver in your sample, assuming that the sample is 100% silver. Weigh out two times the amount
of NaCl calculated (to ensure complete precipitation) and dissolve the sodium chloride in 25 mL of
distilled water. Remove the watch glass from the first beaker, and rinse any moisture on the bottom
of the watch glass back into the beaker with your wash bottle to be sure that no silver is lost. To
precipitate the silver as silver chloride, slowly add the sodium chloride solution to the dissolved silver.
Stir with a stirring rod, and rinse any solution clinging to the rod into the beaker with distilled water.
Gently heat (without boiling) the solution for about 15 minutes. This will cause the precipitate
particles to grow larger so they are easier to filter (while you are waiting go on to step four).
4. Premass the Gooch Crucible (It should be pre-dried before doing this) and set up your funnel similar
to the figure below:
5. Filter the Silver Chloride: Attach the filter crucible to a suction flask. In a 150 or 250 mL beaker, add 1
mL of HNO3 (the stem of a graduated disposable pipet is about 1 mL and will work well here) to 75
mL of distilled water to use as a wash. The addition of the acid to the rinse water helps to keep the
precipitate from "peptizing," or forming extremely small particles that will run through the filter.
Carefully pour the solution of the silver chloride down a stirring rod into the funnel. Using a rubber
policeman, try to “broom” as much of your product into the crucible. Using a water bottle and using
minimal water, wash the rest of the product into the crucible. Using your HNO3 wash and a
disposable pipet, wash your product with four or five pipets full of wash. This will assure that all of
the soluble copper, sodium, and extra chloride ions separate themselves from our insoluble silver
chloride product.
6. Dry the Silver Chloride and Determine Its Mass: Put the Gooch Crucible with silver chloride in the oven
and let it dry for at least 2-3 hours(a good stopping point for the first day). When it is dry and cool,
find its mass on the analytical balance.
7. Calculate the Percent Silver in the Alloy: From your data, determine the percent of silver in your sample.
Please be careful to use strict quantitative techniques in this lab:
- Rinsing the liquid on a stirring rod and on a watch glass back into the beaker
- Pouring a solution down a stirring rod into the filter funnel
- Removing all of the precipitate using a policeman
- Washing the precipitate free of foreign ion
CALCULATIONS:
In your calculation section show all of the work needed to find the percentage of silver in your silver
alloy. Research the accepted value for the percent of silver in sterling silver (cite your source).
Calculate the % error of your experiment based upon the above two values. Please be detailed in
your analysis.
QUESTIONS:
1. Provide at least three possible sources of error that would cause the experimental percentage of silver
to differ from the true percentage. For each error discussed, indicate whether it would cause the
experimental results to be too small or too large.
2. Why was a twofold excess of chloride added to precipitate the silver?
3. Why was it necessary to wash the precipitate?
APPENDIX A:
The reaction between silver and nitric acid and copper and nitric acid can be summarized below:
3Ag(s) + 4HNO3(aq)
3Cu(s) + 8HNO3(aq)
→
→
3AgNO3(aq) + NO(g) + 2H2O(l)
3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)
The nitrogen monoxide produced then reacts with oxygen in the air to form brown , poisonous NO2:
2NO(g) + O2(g)
→
2NO2(g)